1. Field of the Invention
The invention relates to methods and systems for controlling the operation of xe2x80x9clean-burnxe2x80x9d internal combustion engines used in motor vehicles to obtain improvements in vehicle fuel economy.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO2) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (8), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as NOx, based, for example, on detected values for instantaneous engine speed and engine load.
To limit the amount of feedgas constituents, such as HC, CO and NOx, that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrap.xe2x80x9dBoth the upstream and downstream catalyst store NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and releases previously-stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry.
Under one prior art approach, the duration of any given lean operating excursion (or its functional equivalent, the frequency or timing of each purge event) is controlled based upon an estimate of how much NOx has accumulated in the trap since the excursion began. For example, in U.S. Pat. No. 5,473,887 and U.S. Pat. No. 5,437,153, a controller seeks to estimate the amount of NOx stored in the trap by accumulating estimates for feedgas NOx which are themselves obtained from a lookup table based on engine speed, or on engine speed and load (the latter perhaps itself inferred, e.g., from intake manifold pressure). The controller discontinues the lean operating excursion when the total feedgas NOx measure exceeds a predetermined threshold representing the trap""s nominal NOx-storage capacity. In this manner, the prior art seeks to discontinue lean operation, with its attendant increase in engine-generated NOx, before the trap is fully saturated with NOx, because engine-generated NOx would thereafter pass through the trap and effect an increase in tailpipe NOx emissions.
With the trap thus deemed to have been xe2x80x9cfilledxe2x80x9d with NOx, the prior art teaches the immediate switching of the engine operating condition to a rich engine operating condition characterized by combustion of an air-fuel ratio that is substantially rich of the stoichiometric air-fuel ratio. The rich operating condition is continued, for example, for either a fixed time period sufficient to purge the trap of all stored NOx, or until a downstream oxygen sensor indicates the xe2x80x9cbreak-throughxe2x80x9d of rich exhaust gas, thereby signaling the release from the trap of all stored NOx.
Because of the risk of emissions break-through if the trap is over-filled, the prior art teaches an initialization procedure at engine start-up characterized by the immediate purging of the trap of any stored NOx. Accordingly, immediately upon engine start-up, the controller selects the trap-purging rich engine operating condition and continues to so operate the engine until the trap is confirmed to be empty of stored NOx, either by running rich for a predetermined minimum time period, or until rich exhaust gas is detected downstream of the trap. As a result, each engine start-up incurs an immediate fuel economy penalty.
It is an object of the invention to control the operation of a lean-burn internal combustion engine at start-up so as to reduce any fuel economy penalty associated with trap initialization.
In accordance with the invention, a method is provided for controlling the operation of a lean-burn internal combustion engine, the exhaust gas from which is directed through an exhaust purification system including a lean NOx trap which stores an exhaust gas constituent when the exhaust gas is lean and releases previously-stored exhaust gas constituent when the exhaust gas is rich. Under the invention, the method includes determining a first measure representing an amount of the first exhaust gas constituent stored in the device at a time when the engine is shut off; and enabling lean engine operation upon an immediately-subsequent engine start-up based on the first measure. In an exemplary embodiment, the method also includes determining a second measure representing the amount of the first exhaust gas constituent stored in the device at the time of the subsequent engine start-up, wherein the second measure is based at least in part on the first measure and a temperature of the device; and downwardly adjusting the first measure based on the length of time.
In accordance with another feature of the invention, the enabling step preferably includes determining an amount of fuel, in excess of a stoichiometric amount of fuel, required to release substantially all of the previously-stored amount of the exhaust gas constituent based on the first measure; and prohibiting lean engine operation until the engine has been operated at a rich operating condition sufficient to add the excess fuel amount to the exhaust gas passing through the device.
Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.